Access window variable from Content Script

I have a Chrome Extension that is trying to find on every browsed URL (and every iframe of every browser URL) if a variable window.my_variable_name exists.

So I wrote this little piece of content script :

function detectVariable(){
    if(window.my_variable_name || typeof my_variable_name !== "undefined") return true;
    return false;
}

After trying for too long, it seems Content Scripts runs in some sandbox.

Is there a way to access the window element from a Chrome Content Script ?